Symmetrical Stretch

There is an easy way to realize the two-way stretch which is adding two actions of stretch. Though it can realize a two-way stretch, the action of stretch is independent. The parameter needs some additional setup to realize the two-way symmetrical stretch.

First, add two stretch actions for parameter, and select the two grips of the two parameters as the relevant point of each action.

Second, in the misc, modify the base location from "Start Point" to "Mid Point", like shown in the picture below. Save and exit the block editor.

For the convenience of watching the effect, we drew a vertical center line in the middle of the keyway, and then stretched the grip to the left side of the keyway or the right side, as shown in the picture below. As the grips are moving, the two-way symmetrical stretch is realized.